Many older pieces for horn were written for a horn not keyed in F as is standard today. As a result, a requirement for modern horn players is to be able to read music in other keys. This is most commonly done by transposing the music "on the fly" into F. A reliable way to transpose is to liken the written notes (which rarely deviate from written C, D, E, and G) to their counterparts in the scale the F horn will be playing in. Commonly seen transpositions include:
- B♭ alto – up a perfect fourth 1
- A alto – up a major third
- G – up a major second
- E – down a minor second
- E♭ – down a major second 2
- D – down a minor third
- C – down a perfect fourth
- B♭ basso – down a perfect fifth 1
Some less common transpositions include:
- A♭ alto – up a minor third
- G♭ – up a minor second
- D♭ – down a major third (used in some works by Berlioz, Verdi and Strauss (Der Rosenkavalier))
- B – down a tritone (used by Brahms) 3
- A basso – down a minor sixth (used in some works by Verdi)
- A♭ basso – down a major sixth (used in some works by Verdi)
- G basso – up a minor seventh (used in some works by Verdi)
It has been speculated that one of the reasons Brahms wrote for horn in the awkward key of B was to encourage the horns to use the natural horn; he did not like the sound of the new valved horns. An example of this is when Brahms picked the second horn player, Wilhelm Kleinecke, in the Vienna Opera for a performance of the Horn Trio in E flat, op. 40 over the first horn, Richard Lewy, because Lewy only played the valved horn. 
Sometimes it is ambiguous whether a piece should be transposed up or down (i.e. B♭ alto versus B♭ basso when only B♭ is written). It is usually safe to assume that the most common and reasonable transposition is the intended transposition (i.e. it stays in the normal horn range). More can be decided from the history of the composer. For example, Verdi and other opera composers used many low and odd transpositions. For Haydn symphonies that have trumpets, the lower transposition for the horns is usually correct; otherwise the high transposition is usually correct. After gaining much experience, this decision-making should come easily to the horn player.
1In older scores (many times German), B♭ alto and basso are written simply as "B."
2E♭ horns were used extensively in military bands in the early 20th century, therefore band parts written for chromatic E♭ horns are common.
3Brahms indicated the key of B as "H."
Learning to Transpose
Many ideas are out there as to how to think of transposing. Here a few are presented. Pick the ones that work for you and use them. In the end, knowing what note to play is what is important.
Note by Note
Once you figure out how far you need to transpose, just think that in your head for every note. For example, for D horn you need to transpose every note down a minor third (three half steps). When you come to a third space C, you think a minor third below that and play an A.
Pretend the written music has a different clef.
- B♭ Horn - If you have a double horn, as most people do, you can use F horn fingerings on the B♭ side of the horn. This does not work for all fingerings, but if you are in doubt, it can come in handy.
- C Horn - This is similar to B♭ horn, but instead use B♭ fingerings on the F side of the horn. Again this is not fool-proof and does not work for all fingerings.
Old Notation was the practice of notating bass clef Horn parts a fourth BELOW sounding pitch instead of a fifth ABOVE sounding pitch, which is the standard today.
These parts should be played an octave above the written pitch. A good indicator of old notation is the presence of a pedal C (C2) or lower.
In modern composition, old notation should never be used. It is outdated and unnecessary.